function A(a){
  this.a = a;
}

// B.prototype = new A(1)
B.__proto__ = A
B.prototype.constructor = B

function B(a,b){
  A.apply(this,arguments)
  this.b = b
}


let b = new B(1,2)

console.log(b.__proto__)
console.log(B.prototype)
console.log(B.__proto__)
console.log(A.prototype)
console.log(A.__proto__)


// B {}
// B {}
// [Function: A]
// A {}
// [Function (anonymous)]
